Synthetic resin container having improved shape stability

ABSTRACT

The synthetic resin container according to the present invention has a waist dividing a container main body portion into upper and lower parts, wherein the waist is formed on an annular groove surrounding the main body portion to as to be convex toward the interior of the container. The annular groove has reinforcing ribs with a level higher than a groove bottom of the annular groove and lower than the surface of the main body portion. The container main body portion includes reinforcing lateral ribs each having a concave portion which is positioned at the same level as a surface of the container, which or forms a slight step relative to the surface of the container. The main body portion has a plurality of ridges converging toward the associated central convergent point, respectively, thereby defining multi-faceted concave walls inclined toward the associated convergent points, respectively.

BACKGROUND ART

1. Technical Field

The present invention relates to a thin-walled synthetic resincontainer, and intends to provide a thin-walled synthetic resincontainer capable of effectively avoiding lowering of the rigidity ofthe container, which tends to be caused by its thin-walled nature, tothereby exhibit a required shape stability of the container.

2. Related Art

Synthetic resin containers, such as PET bottles made of polyethyleneterephthalate resin, have been widely used as containers, e.g., forfilling therein foods, beverages, cosmetics or medicines since suchcontainers are light in weight and can thus be easily handled, havetransparency to exhibit a refined appearance comparable to glasscontainers, and can be produced at low cost.

This type of synthetic resin container has a relatively low mechanicalstrength against external forces. Therefore, when the container isgripped at its main body portion for pouring the content out of thecontainer, the container inevitably undergoes deformation at its grippedportion. It is thus a typical countermeasure to appropriately controlthe container wall thickness and form reinforcing means, e.g.,longitudinal ribs, lateral ribs or waists (i.e., circumferential groovessurrounding the main body portion), for improving the resistances of thecontainer to external forces, such as buckling strength and rigidity.

Furthermore, there is an increasing demand for thin-walled (orlight-weighted) containers so as to reduce the resin amount to be usedper one container from a standpoint of effective utilization ofresources and reduction in the amount of wastes, resulting in asituation where the rigidity of the container is inevitably furtherlowered to deal with such a demand. In this instance, particularly inthe case of a container having a polygonal cross-section and formed witha waist, the container tends to be deformed in its cross-section intorhombic shape due to the thin-walled nature of the entire container,when external force is applied to the waist portion in a diagonaldirection at the corner of the waist portion. From such a viewpoint, inconnection with a waist-formed synthetic resin container, there is astrong demand for a container structure having higher buckling strengthand rigidity, and capable of minimizing deformation in terms of itsouter shape of the container even when it is made thin-walled.

Meanwhile, synthetic resin containers have a relatively low thermalstrength, and particularly, containers made of PET resin (polyethyleneterephthalate resin) have a limitation on the filling temperature ofcontents, which must be not higher than approximately 85 to 87° C. Thus,when the contents at temperatures exceeding such a temperature range isfilled into the containers, the containers are inevitably deformed dueto heat shrinkage thereof. In this respect, there is known a technologyas disclosed in JP 7-67732 B2, for example, for improving the heatresistance of containers by carrying out at least two times ofbiaxial-stretching blow molding before and after an intermediate heattreatment step, and there is indeed a tendency to raise the allowablefilling temperatures of contents.

However, when this type of targeted container is thin-walled (orlight-weighted) so as to reduce the used resin amount (for example, whenthe used resin amount is reduced from approximately 69 grams to 55 gramsor less, in the case of a 2-liter container), the lower region of thecontainer main body portion tends to bulge outwardly due to theself-weight (i.e., hydraulic head) of the contents and due to theaffection of heat of the contents, thereby making it difficult to retainthe initial shape of the container. Such bulging is particularly markedin containers having pressure-reduction absorbing panels, which serve tocompensate for the shape deformation of the container due to pressurereduction within the container.

Although it is effective to form lateral ribs on a container main bodyportion so as to retain the outer shape of the container, the ribs maywarp due to affection of heat because the container is thin-walled,thereby failing to effectively exhibit the reinforcing function of theribs. From such a viewpoint, in connection with a synthetic resincontainer having an improved heat resistance allowing a hot filling ofthe contents at a relatively high temperature, there is a strong demandfor a container structure having an excellent shape stability capable ofretaining the initial shape of the container regardless of itsthin-walled structure.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide asynthetic resin container capable of solving the above-mentionedproblems of the prior art and effectively avoiding lowering of therigidity of the container regardless of its thin-walled nature, tothereby exhibit a required shape stability of the container.

According to a first aspect of the present invention, there is provideda synthetic resin container provided with at least one waist, whichdivides a main body portion of the container into upper and lower parts,wherein the waist comprises an annular groove surrounding the main bodyportion so as to be convex toward the interior of the container, and theannular groove is provided with reinforcing ribs each having a levelhigher than a groove bottom of the annular groove and lower than thesurface of the main body portion.

Preferably, the main body portion of the container has a polygonalcross-section, and each of the reinforcing ribs is arranged in a regionwhich extends beyond an associated one of corners of the polygonalcross-section.

Preferably, each of the reinforcing ribs has an arcuate shape at itsouter periphery.

According to a second aspect of the present invention, there is provideda synthetic resin container obtained by biaxial-stretching blow molding,wherein the container has a main body portion provided with reinforcinglateral ribs each having a concave portion which is positioned at thesame level as a surface of said container, or which forms a slight steprelative to said surface of said container.

Preferably, the concave portions are formed at central regions of thelateral ribs, respectively.

Preferably, the lateral ribs are projected inwardly of the main bodyportion of the container. Each of the lateral ribs may have such alength that the opposite ends of the lateral rib are short of theassociated pillars, respectively.

Preferably, the synthetic resin container according to the presentinvention is provided with pressure-reduction absorbing panels at themain body portion.

Preferably, the synthetic resin container according to the presentinvention is provided with longitudinal ribs projected inwardly of themain body portion. The longitudinal ribs may have concave portionsaround the longitudinal ribs themselves, respectively, wherein theconcave portions are lower than a surface of the container main bodyportion.

Preferably, the synthetic resin container according to the presentinvention has a quadrilateral cross-section including at least fourlocations around the main body portion, in the form of pillarscomprising longitudinally elongated concave or convex surfaces,respectively, extending along a main axis of the container.

According to a third aspect of the present invention, there is provideda synthetic resin container obtained by biaxial-stretching blow molding,wherein the synthetic resin container has a main body portion providedwith a plurality of ridges converging toward the associated centralconvergent points, respectively, such that the ridges form multi-facetedconcave walls inclined toward the associated convergent points,respectively.

Preferably, the multi-faceted concave walls define thepressure-reduction absorbing panels. Each of the pressure-reductionabsorbing panels may exhibit a quadrilateral shape, and the associatedribs of the quadrilateral shape may start from four corners of thequadrilateral shape to converge at the associated central convergentpoint. Preferably, each of the central convergent points has a lateralgroove oriented perpendicularly to a main axis of the container.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail hereinafter,with reference to the preferred embodiments shown in the drawings.

FIG. 1 is a front view of a waist-formed synthetic resin containeraccording to a first embodiment of the present invention.

FIG. 2( a) and FIG. 2( b) are a plan view and a bottom view,respectively, of the container of FIG. 1, and FIGS. 2( c) through (i)are cross-sectional views taken along line c-c through line i-i of FIG.1, respectively.

FIG. 3 is a front view of the reinforcing rib in the container of FIG.1.

FIG. 4 is an enlarged view of the essential portion of the containershown in FIG. 1.

FIG. 5 is a front view of a synthetic resin container according to asecond embodiment of the present invention.

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 5.

FIG. 7 is a front view of a synthetic resin container according to athird embodiment of the present invention.

FIG. 8 is a front view of a synthetic resin container according to afourth embodiment of the present invention.

FIG. 9 is a cross-sectional view taken along line 9-9 of FIG. 8.

FIG. 10 is a view showing an essential portion of the pressure-reductionabsorbing panel.

FIG. 11 is a front view of a synthetic resin container according to afifth embodiment of the present invention.

FIG. 12 is a cross-sectional view taken along line 12-12 of FIG. 11.

FIG. 13 is an enlarged view of the pressure-reduction absorbing panel inthe container of FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 through FIG. 4 show a synthetic resin container according to afirst embodiment of the present invention. This container has a fillingvolume of 2.0 liters and is formed in a substantially quadrilateralcross-sectional shape. Reference numeral 11 denotes a waist whichdivides a main body portion of the container into upper and lower parts.This waist 11 comprises an annular groove 11 a surrounding the main bodyportion in a manner to become convex toward the interior of thecontainer.

Reference numerals 12 denotes reinforcing ribs, respectively, eachhaving has a level higher than a groove bottom of the annular groove 11a and lower than the surface of the main body portion. Each reinforcingrib is formed into an arcuate shape at its outer periphery. Thesereinforcing ribs 12 are provided at four corners of the main bodyportion of the container in the present embodiment, respectively.

Although the waist formed by simply recessing the container main bodyportion and thereby dividing the main body portion into upper and lowerparts is provided for the purpose of improving the rigidity of thecontainer, the thin-walled container has a reduced strength at thatregion and thus tends to buckle when applied with a load from the upperor bottom portion of the container, besides that the thin-walledcontainer is easily depressed when gripped at the waist portion.

FIG. 3 is a front view of the reinforcing rib 12 shown in FIG. 1. Whensuch a reinforcing rib 12 is provided at the waist 11, the reinforcingrib 12 functions as a frame of the container, thereby resulting in anextremely restricted deformation of the container main body portion upongripping the same, and also resulting in a remarkably improved bucklingstrength of the container.

Each reinforcing rib 12 is preferably formed into a single arc, so as toavoid stress concentrations and stabilize the outer shape of thecontainer. As can be appreciated from FIG. 4 showing the essentialportion of the outer periphery of the reinforcing rib 12 in enlargedscale, the reinforcing rib 12 has a level L₂ lower than the surfacelevel L of the container main body portion and higher than the groovebottom level L₁ of the annular groove 11 a, so as to effectively exhibitthe function of the reinforcing rib 12. Furthermore, the width of thereinforcing rib 12 in the circumferential direction (i.e., around themain body portion) is such that each end portion of the rib extendsbeyond the associated corner portion of the container and reaches thewaist portion positioned at the walled surface of the container.

Although the above embodiment has been described in connection with astructure wherein the reinforcing ribs 2 are provided for the containerhaving a quadrilateral cross-sectional shape, the present invention isnot limited to the illustrated cross-sectional shape. Namely, thepresent invention is also applicable to containers having a polygonalcross-section, such as rectangular, pentagonal or hexagonalcross-section, as well as to containers having a circular cross-section.The present invention is also applicable to containers having a fillingvolume of not more than 500 milliliters, 1.0 liter, 1.5 liter and evento large-sized containers having a filling volume exceeding 2.0 liters,in addition to the illustrated container of 2.0 liter. There is noparticular limitation in terms of the filling volume.

It is possible to use a thermoplastic resin such as a polyethyleneterephthalate resin as the resin material for the container, and toproduce the container by blow molding a preform obtained by extrusionmolding or injection molding of such a resin.

The container produced by blow molding can be used for either normaltemperature filling or high temperature filling of the contents.Particularly, in the case of containers to be filled with a hightemperature liquid as the contents, it is possible to utilize a normalmolding method for completing the container by performing one time ofbiaxial-stretching blow molding, and another molding method forcompleting the container having an improved heat resistance byperforming at least twice of biaxial-stretching blow molding before andafter an intermediate heat treatment step. Then, any of such containersare allowed to have an improved strength by providing reinforcing ribs12 at the waist, if such waist is provided at the container main bodyportion.

According to the embodiment described above with reference to FIG. 1through FIG. 4, the waist 11 is constituted of the annular groove 11 asurrounding the container main body portion so as to be convex towardthe interior of the container, and the reinforcing ribs 12 are providedsuch that each reinforcing rib 12 has a level higher than the groovebottom of the annular groove 11 a and lower than the surface of the mainbody portion and each reinforcing rib is formed into an arcuate shape atits outer periphery. It is therefore possible to minimize thedeformation of the container upon gripping the waist portion, and toremarkably improve the buckling strength and rigidity of the containereven when the container is thin-walled.

FIG. 5 and FIG. 6 show a synthetic resin container according to a secondembodiment of the present invention. Reference numeral 21 denotes acontainer body, and reference numeral 22 denotes a mouth portionintegral with the container body 21. Furthermore, reference numeral 23denotes a groove portion for dividing the container body 21 into upperand lower parts to thereby enhance the rigidity of the container, andreference numerals 24 denotes pressure-reduction absorbing panels,respectively. Each pressure-reduction absorbing panel 24 has a functionfor preventing a shape deformation of the container due to a volumechange thereof as a result of cooling of the contents therein.

Reference numeral 25 denotes reinforcing lateral ribs formed at the mainbody portion of the container so as to extend across thepressure-reduction absorbing panels 24, respectively. Each lateral rib25 has a concave portion 25 a at a central region (i.e., the centralregion in the longitudinal direction) of the lateral rib itself, suchthat the concave portion is flush at a position 25 a 1 with a surface ofthe container main body and forms a slight step relative to the surfaceof the container main body portion at a position 25 a 2.

Reference numerals 26 denotes reinforcing longitudinal ribs alternatelyarranged between the lateral ribs 25, respectively, and referencenumerals 27 denotes pillars formed at four locations around the mainbody portion. Each pillar 27 has a longitudinally elongated concavesurface 27 a formed into a polygonal line shape and extended along amain axis P of the container.

There is known a synthetic resin container formed by one time ofbiaxial-stretching blow molding, or another synthetic resin containerformed by at least two times of biaxial-stretching blow molding beforeand after an intermediate heat treatment step, such as that disclosed inJP-7-67732 B2. In this type of container, the residual stress in thecontainer main body portion is remarkably mitigated and the strengthagainst external heat is enhanced by virtue of the increased density ofthe resin. However, even if lateral ribs are provided to ensure theshape stability of this type of container when the container isthin-walled to reduce the resin amount to be used per one container, thelateral ribs inevitably tend to warp due to the self-weight (hydraulichead) of the contents themselves and due to the affection of the heatpossessed by the contents. In such instance, the lateral ribs do notrestore due to the plastic deformation of the lateral ribs themselveseven after cooling of the contents, thereby resulting in a poorappearance of the container. According to the embodiment of FIG. 5 andFIG. 6, however, each concave portion 25 a provided at the associatedlateral rib 25 is flush at the position 25 a 1 with the surface of thecontainer and forms a slight step relative to the surface of thecontainer at position 25 a 2, so as to prevent warpage of the lateralrib 25 as a whole and thereby retain the initial shape of the container.Further, the lateral rib 25 effectively exhibited the intended functionto keep the container in a highly rigid state. It is preferred for thelateral ribs 25 to be arranged along the widthwise direction of thepressure-reduction absorbing panels 24, respectively, so as to extendacross these panels.

Although each lateral rib 25 has been exemplarily shown in FIG. 5 tohave such a length that the opposite ends of the lateral rib reach theassociated pillars 27, respectively, the length of the lateral rib maybe preferably short of the pillars 27 so as not to affect the functionof the pillars 27. Further, each pillar 27 is preferably constituted tohave the concave surface 27 a formed into the polygonal line shape or aconvex surface 27 a in an R shape, such that the pillar 27 does noteasily buckle even upon application of a load from the upper or lowerportion of the container.

The longitudinal ribs 26 may be arranged between the lateral ribs 25 andadjacent to the pillars 27, respectively. Provision of such longitudinalribs 26 ensures that, even when the container is to be deformed due to aload upon gripping the container, the deformation of the containeralways occurs at constant locations i.e., in the directions of the endportions of lateral ribs 25, in the present embodiment, so that thecontainer is immediately restored to its initial shape upon releasing ofthe load that caused the deformation. This means that it is possible toimprove the restoring performance of the container after deformation.

FIG. 7 shows a synthetic resin container according to a third embodimentof the present invention. In this embodiment, the region around eachlongitudinal rib 26 is formed as a concave portion 28 which is lowerthan surface of the container main body portion such that the contourshape of the longitudinal rib 26 is embossed upon molding the containerto thereby further enhance the reinforcing effect near the cornerportion of the container, while each lateral rib 25 is made to have areduced length such that the opposite ends thereof are short of theassociated pillars 27, respectively. Such a constitution ensures thatthe buckling strength is further enhanced in the container having aquadrilateral cross-section, and the restoring ability of the containerafter deformation is further improved.

When containers are produced by adopting a polyethylene terephthalateresin as the resin for the container and conducting two times ofbiaxial-stretching blow molding before and after an intermediate of heattreatment step, the following procedure shall be followed.

First of all, a preform obtained by extrusion molding or injectionmolding is heated to a temperature which allows exhibition of stretchingeffect, e.g., to a temperature range of 70 to 130° C., and morepreferably 90 to 120° C. Then, the first time of biaxial-stretching blowmolding is conducted under a temperature condition of 50 to 230° C.,more preferably 70 to 180° C., with a surface stretching ratio of 4 to22 (more preferably 6 to 15, into an oversized intermediate body havinga volume which is about 1.2 to 2.5 times that of the finishedcontainer). Next, the thus obtained blow molded body is applied with aforced heat treatment at a temperature in a range of 110 to 255° C.,more preferably 130 to 200° C., so as to be shrunk to a size which isabout 0.60 to 0.95 times that of the finished container, to therebyremove the residual stress in the article. Subsequently, there isconducted a second time of biaxial-stretching blow molding at atemperature in a range of 60 to 170° C., more preferably 80 to 150° C.It is noted that the container according to the present invention may beof course molded by one time of biaxial-stretching blow molding, withoutfollowing the above conditions.

In this way, according to the embodiment shown in FIG. 5 and FIG. 6 orthe embodiment shown in FIG. 7, the resin container having an improvedheat resistance is provided with the reinforcing lateral ribs 27 havingthe concave portions 27 a, respectively, each of which is positioned atthe same level as the surface of the container or forms a slight steprelative to the surface of the container, thereby making it possible tomaintain an improved shape stability even when the container isthin-walled for reducing the used amount of resin.

FIG. 8 through FIG. 10 show a synthetic resin container according to athird embodiment of the present invention. Reference numeral 31 denotesa container body, reference numerals 32 denotes reinforcing lateralribs, respectively, appropriately formed at the main body portion of thecontainer body 31, reference numerals 33 denotes reinforcinglongitudinal ribs, respectively, appropriately formed at the main bodyportion of the container body 31, and reference numerals 34 through 39denote pressure-reduction absorbing panels, respectively, shown as beinglinearly arranged on the main body portion of the container body 31 byway of example.

While the panels 36, 37 among the pressure-reduction absorbing panels 34through 39 are shown as having flat surfaces, respectively, each of theremaining panels 34, 35, 38, 39 is provided with ridges R (inwardlyconvexed ridges) converging at a central convergent point of theapplicable panel so that the ribs R define a multi-faceted concave wallcomprising wall surfaces 34 a through 34 d, 35 a through 35 d, 38 athrough 38 d or 39 a through 39 d, which are inclined toward theassociated convergent point Ro. The details of the panels 34, 35, 38, 39are shown in FIG. 10.

By forming the pressure-reduction absorbing panels 34, 35, 38, 39 intothe multi-faceted concave walls according to the embodiment of FIG. 8through FIG. 10, respectively, it is possible for the ridges R to act asreinforcing frames of the panels, respectively, thereby advantageouslyavoiding bulging of the container due to the hydraulic head of thecontents. Further, since the shape deformation of the container due tothe pressure reduction is compensated for by the entirety of eachpressure-reduction absorbing panels 34, 35, 38, 39, this function is notaffected by the associated ridges R.

Although the pressure-reduction absorbing panels 36, 37 are embodied tohave flat surfaces in the embodiment of FIG. 8, such an arrangement isto stabilize the shape of the container, and it is possible in thepresent invention to constitute the container by appropriately combiningpanels having flat surfaces, with panels having multi-faceted concavewalls.

FIG. 11 through FIG. 13 show a synthetic resin container according to afourth embodiment of the present invention. This embodiment is achievedwhen the convergent point R₀ of each of the pressure-reduction absorbingpanels 34, 35, 38, 39 in the embodiment of FIG. 8 through FIG. 10 isprovided with a lateral groove 40 oriented perpendicularly to the mainaxis P of the container. The provision of such lateral grooves 40 allowsa further suppression of bulging of the pressure-reduction absorbingpanels 34, 35, 38, 39 due to the hydraulic head of the contents.

Although the embodiment shown in FIG. 11 through FIG. 13 has beendescribed with reference to an arrangement wherein the multi-facetedconcave walls are applied to the pressure-reduction absorbing panels 34,35, 38, 39 having a reduced wall thickness, such multi-faceted concavewalls can be directly provided at the main body portion of the containerbody 31, without limited to the application to the pressure-reductionabsorbing panels only.

According to the embodiment of FIG. 11 through FIG. 13, the main bodyportion of the synthetic resin container is provided with multipleridges converging toward the associated central convergent points,respectively, such that the ridges define multi-faceted concave wallsthat are inclined toward the associated convergent points, respectively.Therefore, it is possible to retain a high shape stability of a resincontainer having an excellent heat resistance, even when the containeris thin-walled to reduce the used amount of resin.

It will be appreciated from the foregoing description that, according tothe present invention, it is possible to solve various problems of theprior art and realize a thin-walled synthetic resin container capable ofeffectively avoiding lowering of the rigidity of the container due toits thin-walled nature, to thereby exhibit a required shape stability ofthe container.

It is needless to say that the present invention is not limited to theabove-mentioned embodiments, and may be carried out with numerousvariants.

1. A synthetic resin container provided with at least one waist, whichdivides a main body portion of the container into upper and lower parts,wherein: said main body portion has a polygonal cross-section includingfour corner portions around said main body portion, said corner portionsbeing a pillar comprising a longitudinally elongated nonplanar surfaceextending along a main axis of said container, said waist comprises anannular groove surrounding said main body portion so as to be convextoward the interior of said container, said annular groove is providedwith reinforcing ribs each having a level higher than a groove bottom ofsaid annular groove and lower than a surface of said main body portion,and each of said reinforcing ribs is arranged at a corner portion of themain body portion, each reinforcing rib having a width in thecircumferential direction such that each end portion of each rib extendsbeyond an associated corner of the polygonal cross-section and intoneighboring side surfaces of the waist.
 2. The synthetic resin containeraccording to claim 1, wherein each of said reinforcing ribs extendsbetween said neighboring side surfaces as a single arc, as seen in thepolygonal cross-section.
 3. The synthetic resin container according toclaim 1, wherein each of said reinforcing ribs has an accurate shape atits outer periphery.
 4. A synthetic resin container obtained by abiaxial-stretching blow molding, comprising: a main body portionprovided with reinforcing lateral ribs each having a concave portionwhich is flush at a first position with a surface of said container, andforms a slight step relative to said surface of said container at asecond position, wherein: said main body portion has a quadrilateralcross-section including at least four locations around said main bodyportion, each location being a pillar comprising longitudinallyelongated nonplanar surfaces, respectively, extending along a main axisof said container, each of said lateral ribs has a length such that theopposite ends of the lateral rib are short of the associated pillars,respectively, and said main body portion is provided with longitudinalribs located between said lateral ribs and adjacent to said pillars. 5.The synthetic resin container according to claim 4, wherein said concaveportions are formed at central regions of said lateral ribs,respectively.
 6. The synthetic resin container according to claim 4,wherein said lateral ribs are projected inwardly of said main bodyportion of said container.
 7. The synthetic resin container of claim 4,wherein said synthetic resin container is provided withpressure-reduction absorbing panels at said main body portion.
 8. Thesynthetic resin container according to claim 4, wherein saidlongitudinal ribs projected inwardly of said main body portion.
 9. Thesynthetic resin container according to claim 8, wherein saidlongitudinal ribs have concave portions around said longitudinal ribsthemselves, respectively, and said concave portions are lower than asurface of said main body portion of said container.
 10. A syntheticresin container obtained by biaxial-stretching blow molding, comprising:a main body portion provided with a plurality of ridges convergingtoward associated central convergent points, respectively, such thatsaid ridges form multi-faceted concave walls inclined toward theassociated convergent points, respectively, the main body portionprovided with a plurality of longitudinal ribs, wherein each of saidcentral convergent points has a lateral groove oriented perpendicularlyto a main axis of said container.
 11. The synthetic resin containeraccording to claim 10, wherein said multi-faceted concave walls define apressure-reduction absorbing panel.
 12. The synthetic resin containeraccording to claim 11, wherein said pressure-reduction absorbing panelexhibits a quadrilateral shape, and the associated ridges of saidquadrilateral shape start from four corners of said quadrilateral shapeto converge at the associated central convergent point.